Cathode-ray tube for color television



March 20, 1956 o, LAWRENCE 2,739,260

CATHODE-RAY TUBE FOR COLOR TELEVISION Filed March 20, 1950 3Sheets-Sheet l SCANNING OSCILLATORS I c Q 5 I E a O. 3 Y o z 3 N g 5 [LI.J LIJ INVENTOR. ER/VfST 0. LAWRENCE A TTORNEYS March 20, 1956 E. o.LAWRENCE 2,739,260

CATHODE-RAY TUBE FOR COLOR TELEVISION Filed March 20, 1950 5Sheets-Sheet 2 &

O GREEN PHOSPHOR A RED PHOSPHOR El BLUE PHOSPHOR IN VEN TOR. ERNEST 0.LAW/PE NOE 25 A TTORNE'YS March 20, 1956 E. o. LAWRENCE 2,739,260

CATHODE-RAY TUBE FOR COLOR TELEVISION Filed March 20, 1950 3Sheets-Sheet 3 INV EN TOR. ERNEST O. LAWRENCE 4'0. swstl A T TORNE Y5United States Patent 6 CATHODE-RAY TUBE FOR COLOR TELEVISION Ernest 0.Lawrence, Berkeley, Calif., assignor, by mesne assignments, to ChromaticTelevision Laboratories, Inc., New York, N. Y., a corporation ofCalifornia Application March 20, 1950, Serial No. 150,732

4 (Ilaims. (Cl. 313-73) This invention relates to cathode-ray tubes fordisplaying television images in polychrome, and particularly to tubesfor displaying such images directly upon the luminescent screen ortarget of the tube by either a two-color or three-color additive systemwithout the necessity for interposing between the target and theobserver any optical system for superimposing the images or portions ofimages which are representative of the primary colors employed in thesystem.

Among the objects of the invention are to provide a cathode-ray tubewherein the color displayed upon any elementary area of the displayingsurface can be controlled by purely electrical means; to provide a tubeof the character described wherein the potentials employed for thecontrol of the color of the display are but a few percent of the totalpotential used for accelerating the electron beam; to provide apolychrome cathode-ray display tube wherein the color instantaneouslyvisible upon the display surface is independent of the path followed bythe beam in scanning the image, so that no means need to be provided forinsuring absolute linearity of scan as in the case where (as has beenproposed in the past) phosphors emissive of ditferent colors are laiddown upon a single display area in strips or spots of sub-elementalsize; to provide a tube of the character described which is applicableto either simultaneous or sequential methods of transmitting the signalsrepresentative of the different primary colors and, among the sequentialsystems, is equally applicable to field sequential, line sequential, dotsequential or dot-sequential multiplex systems; and to provide a tubethe major portion of which is of standard and well known constructionand wherein the luminescent target and the color control appurtenancesthereof are capable of construction within the limits of accuracynecessary to achieve the required results by relatively simple means sothat the overall cost of the completed device is within economicallimits. Monochrome or black-and-white television transmissions in theUnited States are, at the present time, standardized on a basis which isreferred to as a 525-line picture with 2:1 interlace transmitted atthirty frames per second. To accomplish a transmission of this characterthe cathode-ray beam or other scanning element which traces out thetelevision images is deflected in the vertical dimension of the picturefield at 60 cycles per second and in the horizontal direction at 262%times that rate or 15,750 cycles per second, thus producing two fieldsor rasters of scanning lines each containing nominally 262 /2 lines. Thelines of the second raster fall between those of the first so as toproduce a total of 525 horizontal deflections for each two verticaldeflections. Actually the scanning beam is blanked out for approximately8% of the total vertical scanning time to take care of the fly-back orreturn of the beam from bottom to top of the image field, and as aresult the actual picture shows approximately 480 lines in the visiblefield instead of the nominal 525 lines.

Other countries have adopted other standards both for vertical andhorizontal rates of deflection. The tube 2,7392% Patented Mar. 20, 1956of my invention can be constructed and operate under any of thestandards that have so far been proposed in any country, but purely forillustrative purposes and in order to provide a norm against which theresults obtainable in the color tube of this invention can be comparedas to resolution or for other purposes, the United Statesblack-and-white standard will be used and the modifications requiredsatisfactorily to transmit color will be referred to in terms of thenecessary modification of that black-and-white standard.

Theoretically if polychrome television pictures are to be transmitted,with the same amount of detail and resolution in all colors as ispresently utilized in black-and-white, by any three-color additiveprocess three times as much information must be transmitted within agiven interval as is required to produce the monochrome picture.Twocolor processes have been proposed but the color fidelity obtainabletherewith is inferior to the processes using three additive primarycolors. Still greater fidelity could be obtained by the transmissions ofadditional primary colors, but the gain in fidelity is small incomparison to the added complexity and the channel widths required fortheir transmission.

Owing to the large demands for channel space even a channel of threetimes the width required for black and white is presently consideredeconomically unfeasible and therefore various expedients and compromiseshave been adopted to permit the transmission of pictures on a narrowerchannel than that indicated by theory as set forth above. For thepurposes of the present application it is unnecessary and, perhaps,undesirable to go into the expedients that have been adopted for thepurpose mentioned and it is sufficient to state that the systems whichhave been seriously considered have been classified gen erally inaccordance with the manner in which the channels have been dividedbetween the signals representing the various primary colors.

Of these systems there is a general division, first, betweensimultaneous and sequential systems, and, second, a subdivision ofsequential systems as between field-sequential, line-sequential, anddot-sequential systems.

In the various simultaneous systems that have been suggested threeseparate video signals, representative respectively of the red, greenand blue primaries are transmitted. This system is the most profiigateof channel widths and although some saving can be eifected bytransmitting less detail in the red and the blue signals than istransmitted in the green, as of the date of this applicationsimultaneous systems appear to be in abeyance. It is to be noted,however, that the tube of this invention can be used with simultaneoussystems as will hereinafter be described in brief.

The sequential systems diifer primarily in the rate at which the colorrepresented by the signal is changed, all such systems attempting totransmit signals representative of the intensity of the illumination ofeach portion of the picture within the spectral band comprising each ofthe three component primaries in such rapid succession that the colorsare blended in the eye to give the effect of a picture in substantiallyits true colors. As is suggested by the names by which these systemshave become known, in a field-sequential system the entire picture istraced to generate a wave representative of a single primary color, andthen retraced rapidly in each of the other two primary colors insuccession. In general only one of the two fields corresponding to theinterlaced lines of a frame is transmitted before the color is changed,so that, for example, as the odd lines of a complete frame (i. e., thefirst field) are traced in red, the even lines of the first frame or thesecond field are .traced in blue, the odd lines, again, in green, andthe even lines in red, so that, the color cycle progresses,

all lines of the picture are scanned in each of the three colors.

In line-sequential systems successive lines in each field are traced indifferent colors; the fields are interlaced as before and again there isa shift between successive frames so that eventually each line isscanned in different colors. In dot-sequential systems the change fromprimary color to primary color is even more'rapid, the colors changingat a rate which is of the general order of magnitude of that required totransmit a single picture element. Again there is a shift or progressionin the position of the dot representing any individual color, so thatelements of the picture surface which are depicted in the initialscanning in one primary color appear in successive scanning's indifferent color.

In field-sequential systems the method-employedin the past has been touse a rotating color filter in front of the display tube, the latterbeing provided with a phosphor or mixture of phosphors-emissive of allof the primaries. The filter disk or drum removes from the emitted lightthe components of the colors not momentarily being transmitted. Arotating mechanical filter offers a complication which is undesired and,as will be shown, which may be avoided by the use of the tube of thisinvention.

In both line and dot sequential systems the method employed in the pasthas been to form the images representing the three different primarieseither on different tubes or upon different areas of the screen of asingle tube, and to combine the three images optically so that theyappear to be superposed, either by means ofmirrors, semi-silvered ordichroic, or by optical projection methods. Each of these systemsrequires that exact registration of the images be obtained, both withrespect to the electronic scanning of the luminescent target upon whichthey are produced and optically by the instrumentalities used forsuperposing them as they appear to the eye. Again, as will be shownhereinafter, the tube of this invention avoids all of the registrationproblems mentioned; the target or luminescent area is scanned exactly asin the case of a black-and-white image and the color produced by suchscanning is altered by an electrical potential which may be varied atany rate desired, either to produce field, line, or dotsequence.

Broadly considered, the tube of my invention comprises the usualevacuated envelope of glass or glass and'metal which contains a cathoderay gun of any suitable type for directing a beam of cathode raysagainst a target area formed at the end of the tube opposite the gun.The usual deflecting devices, either electrostatic or electromagnetic,for sweeping the beam across the target area in two directions areeither built into the tube or are externally supplied.

A transparent target area may either be formed upon a window in the endof the envelope itself or it may be.

formed upon a sheet of glass or other suitable material closely adjacentto the window. In either case the target area is supplied with a coatingof phosphor which, under bombardment by cathode rays, fluoresces (orphosphoresces with a short period) in one of the primary colors chosenfor the transmission of the polychrome pictures. Closely adjacent tothis coating butinsulated therefrom, is positioned a grating formed of aplurality of mutually insulated conductive strips mounted substantiallyedge-on with respect to the target surface. Each of these strips iscoated, preferably on both sides, with a phosphor emissive of another ofthe primary colors used in the picture transmission; it athree-colorsystemis employed alternate strips carry phosphors emissiveof the two other primaries. Strips carrying phosphors of the same colorare electrically connected, and external connections are provided sothat deflecting potentials can be applied between the strips carryingdifferent color phosphors. The strips forming the grating are inconcentric circles, ellipses or the like.

preferably separated by the distances no greater than.

and preferably less than the width of the lines which form thetelevision image rasters, and the width of the individual strips is ofthe order of ten times their separation, although this value is notcritical and narrower strips can be used at the expense of the use ofhigher potentials to control the colors to be displayed. The alinementof the strips is not important, but they should, in general, beapproximately parallel if this term he considered as broad enough toinclude their disposition The surfaces in which the strips lie should,in general, be parallel to or substantially coincident with a surfaceincludingv the path of the cathode ray as it enters the grating; i'. e.,the strip should lie edgewise to the orifice of the cathoderay gun orelse means should be provided to deflect the cathode ray beam just priorto its entering into the grating to such a degree that in the absence ofany deflecting potential on the grating it will strike only the proximaledge of the strips and will have no appreciable component of velocitynormal to the flat surface of the strips.

With a device of this character, if it be assumed that the primarychosen for emission by the coating of the target surface itself begreen, and that alternate strips of the grating are coated withphosphors which luminesce in red and blue respectively, if the beam bedeflected across the surface of grating and target in the ordinarymanner for scanning a television field, the surface of the target areaitself will receive the entire electron flow from the beam and willluminesce in green as long as no differential potential is appliedbetween the strips forming the grating.

If, however, a differential potential is applied between the adjacentstrips of the grating the beam will be deflected after it enters thespace between the strips toward the one which carries the relativelypositive potential and away from that carrying the negative potential.If the width of the grating strips be ten times their separation apotential difference of approximately 4% of the total voltage used todecelerate the beam will cause all of the electrons of the beam to fallupon the strip which is positive. The strip thus receiving the beam willfluoresce in the primary color proper to the phosphor with which it iscoated, and because the light thus emitted is confined between surfaceswhich are largely reflective a major portion of the emitted light willbe transferred to the target area and will be transmitted through thetranslucent screen. The adjacent strips confine the light thus emittedand reflected to a narrow strip which is approximately equal in width tothe separation between the strips.

It will be seen that since each red strip is between two blue ones itmakes no difference whether the beam happens to fall wholly on one orthe other side of the red strip if the latter is positive the beam willalways be defiected in passing through the grating so that it hits thestrip "which will fluoresce in red. Similarly, each blue strip isadjacent two red ones, and if the blue strips be positive with respectto the red only blue luminescence willbe produced. By making theseparation of the strips less than the theoretical size of theelementary areas of the television picture to be produced, andpreferably of the order of one-half the size, a beam may fall upon thetarget and its grating at random and the right color will always beproduced and in the right spot. Furthermore, it makes no differencewhether the strips forming the grating run parallel to the scanninglines used to form the television picture, normal to those lines, or atsome random angle, the result will always be the same and the rightcolor "produced as long as the deflecting voltage as between thealternate strips is in the proper direction. it is for this reason thatparallelism of the strips is not important, and that the grating can beformed of interlaced spiral coils or otherwise and still obtain thedesired result. There is, of course, some scattering of the lightproduced by the phosphors on the grating. This is of minor importance,however; if the green. phosphor be used to coat the target area itselfas it is well known the eye is most sensitive in the green and mostreceptive to detail carried by the green image. The blue and red maytherefore spread to a limited degree without visible degradation of theimage.

The invention may be better understood by the following detaileddescription considered in conjunction with the accompanying drawings,wherein:

Fig. 1 is a schematic diagram of a tube in accordance with thisinvention, together with elementary circuits illustrative of what isrequired to cause the display thereon to appear in any one of threeprimary colors;

Fig. 2 is a diagram showing the display end of a tube similar to thatshown in Fig. 1 more in detail, but still in schematic form;

Fig. 3 is a view showing the disposition and, to some extent, theconstruction of the grating of the tube of Fig. 2, relative planes ofsection in views 2 and 3 being indicated by the appropriately designatedlines in the two views;

Fig. 4 is an enlarged cross-sectional view indicating one method ofconstructing a grating of the type described;

Fig. 5 is another enlarged detailed view, in perspective, illustratinganother possible method of construction;

Fig. 6 shows stilt another modification of construction of the grating;

Fig. 7 shows a fourth method of constructing the grating;

Fig. 8 is illustrative of a grating formed of concentric spiral stripsrather than the plane parallel strips indicated in other figures;

Fig. 9 shows a grating formed of wedge-shaped strips as distinguishedfrom the plane strips indicated in the preceding figures;

Fig. 10 shows a form of grating wherein the phosphors are deposited onbeads carried on the edges of the grating strips rather than stripsthemselves; and

Fig. 11 is a diagram indicating the deflection of the cathode-ray beamunder different deflecting voltages.

Considering first the tube in general, as shown schematically in Fig. land in slightly greater detail in Figs. 2 and 3, it comprises the usualevacuated envelope 1, which may be of either glass or metal. The tubeis, as is customary, of generally conical or even rectangular form,having at the base of the core a transparent window 3.

Mounted in the smaller end of the cone is an electron gun ofconventional form. This is shown as comprising a filament or heater 5,which raises a thermo-emissive cathode 7 to its emitting temperature.Electrons emitted by the cathode are amplitude modulated by a grid 9 andare successively accelerated by first and second anodes 11 and 13. Theelectron beam thus formed may either be focused by electron lensesestablished by the fields between the various elements or externalfocusing coils (not shown) may be employed. The proper voltages forexciting the various electrodes mentioned are supplied by the televisionreceiver indicated schematically by the block 15.

The electron beam produced by the gun is deflected in two dimensions, inthis instance, by the coils 17 and 19 which carry, respectively,sawtooth currents of the two frequencies utilized to produce thevertical and horizontal scannings as produced by scanning oscillators ofproper form indicated by the block 21. Thus, assuming that the samescanning frequencies are utilized as in the current black-and whitestandards, the coils 17 will carry sawtooth waves having a fundamentalfrequency of 60 cycles per second, while the coils 19 will carrysimilarly shaped waves at a frequency of 15,750 cycles per second. Thecurrents in these coils are adjusted to cause the oathode-ray beam totrace upon the Window of the tube rectangular rasters of the propershape and interlace.

Formed upon the Window of the tube is a translucent layer of a phosphorwhich, when excited by the electron beam, will emit light correspondingto one of the primary colors chosen for use in the system wherein thetube is to be employed. Actually, of course, the particles of thephosphors which are deposited upon the window to form the screen are ofmicroscopic size. They are illustrated in the drawing, however, bydiscrete circles, and as, for reasons which will be gone into more fullyhereinafter, I prefer to deposit the green phosphor directly upon thetarget area of the tube, circles of this type will be used throughoutthe drawings to indicate the green phosphor. Other phosphors used in theconstruction of the device will normally be emissive of red and bluelight respectively, and like the green phosphor will be formed ofparticles of microscopic size. They will be illustrated, however, bysmall triangles and rectangles respectively, such representation, ofcourse, being purely symbolic.

The luminescent coating 23, deposited directly upon the Window 3 isrendered conducting by any of a number of well known methods, perhapsthe best of which is the deposition upon it, on the side facing thecathode-ray gun, of an extremely thin metallic layer such as may beformed, for example, by the evaporation of aluminum. A lead 25 connectsthis coating with the receiver which is arranged to apply suitablepotential thereto to give the cathode-ray beam its final acceleration.

Closely adjacent luminescent target area 23 there is positioned a gridor grating formed of alternate mutually insulated strips or" conductingmaterial coated on both sides with phosphors emissive of one of theother two primaries employed in the system. Strips 27 carrying the redphosphor are connected together, as are the strips 29 which carry theblue phosphor. A lead 31 connects all of the red phosphors to one poleof a switching mechanism 32 which is here shown, for illustrativepurposes only, as being of the mechanical type, while a second lead 33connects all of the blue bearing strips to another pole of the same oran interlocked switch 32'. Means, here illustrated as batteries 35, 35,connect from the lead 25 to the contacts of the switches. The batteries35, 35' are so poled that when the switches are thrown in one directionall of the strips 27 will be positive with respect to the target 23while all of the strips 29 will be negative to the target, whereas, whenthe switches are thrown in the opposite direction, the reverse will bethe case. When the switches are in their intermediate position all ofthe strips and the target area will be at the same potential.

It is to be emphasized that this mechanical switching arrangement isshown merely for illustrative purposes and to indicate the nature anddirection of the potentials employed. In any practical case electronicswitching will undoubtedly be used, but such electronic switchingarrangements are no part of this invention.

The diagram of Fig. l, as well as those which will be described later,are not to be considered as representative to scale of the relativesizes or separations of the strips of the color grating, but are merelya showing of the relative positions of the grating and of the stripscomprising it. Means are provided for causing the electron beam to enterthe grating at any point in a direction parallel to the wider dimensionof the strips, so that when the latter are at the same potential as thetarget area itself they will have no tendency to deflect the beam,which, accordingly, will strike the strips at barely grazing incidenceif at all. This can be accomplished in two different manners; eitherauxiliary focusing means may be used adjacent the display end of thecathode ray tube for bending the beam into a path parallel with thestrips, or the strips can be arranged in planes slightly tilted withrespect to each other and which intersect in a common line substantiallyat the orifice or effective exit aperture of the electron gun, as isindicated in Fig. 1. The two procedures are considered to be equivalent;means for setting up corrective fields of the character first mentionedare well known and therefore arenot here'shown. Arrange ments fortilting the strips as mentioned are illustrated in Figs. and 7.

The relative widths and spacings of the strips forming the grating aresubject to rather wide variations. It can be shown that if the width ofthe strips is ten times their separation, a potential difference of 4%of the voltage employed to give the electrons their velocity uponentering the grating, applied between the two sets of strips, will besufficient to cause all of the electrons of the beam which enter thespace between two adjacent strips to impinge on whichever of the two isthe more positive; i. e., if the voltage accelerating the beam is 10,000volts a 400-volt differential between the two sets of strips. will causeall of the electrons to strike the strips carrying either the blue orthe red phosphors as the case may be.

Substantially all of the phosphors now used commercially are white ornearly so. An electron beam entering the space between the adjacentstrips and striking entirely upon the red or blue phosphors will producelight of the corresponding color and a large portion of this light willfall, either directly or after reflection from the opposing surface,upon a portion of the translucent target area defined between theadjacent strips. There will, it is true, he a considerable loss of lightin this process, but by suitable choice and distribution of phosphorsand accelerating potentials the light which finally passes through thewindow 3 can be made to have the proper radiation intensity and relativeluminosity so that when the beam is modulated in succession by signalsrepresentative of all three primaries the result is effectively a whitelight emitted from the screen and the proper color balance may thus besecured. Owing to the fact that the strips of the grating aresubstantially perpendicular to the main window through which the lightemitted from them is viewed there may be some tendency for the colorbalance to change as the angle of observation changes. This is largelyneutralized by the scattering of the light by the coating of phosphor onthe window, but if this is not sufiicient frosting of the window surfaceprior to the deposition of the phosphor will accomplish the desiredresult.

The ratio of separation of the strips to their widths is not critical,but it is directly connected with the voltage required to cause completeswitching as between the primary colors emitted, this voltage beingproportional to the square of the ratio between separation and width;thus while a separation-width ratio of requires about 4% of the beamaccelerating voltage to cause complete deflection, a ratio of 5% willcause complete color transfer upon application between the strips of 1%of the accelerating voltage. The choice of separation-width ratio istherefore a compromise between electrical and mechanical features and issubject to variation at the option of the designer of the equipment.

The absolute dimensions of the strips constituting the grating are afunction of the size of the screen and of the resolution which is to berequired of it. Preferably the separation between the adjacent stripsshould be materially smaller than the dimension of the picture elementswhich it is desired to resolve. A separation equal to onehalf ofdiameter of a single picture element is satisfactory, but a separationof one-third of the diameter of the picture element to be resolved givessomewhat improved resolution. Further reduction of the separation givesno improvement in resolution if the beam diameter is of the proper size.

What this means in terms of atubedesigned to produce color picturesconforming to the present 525" line blackand-white standard can readilybe shown; a cathode ray tube having a window 16 inches in diameter willshow a picture roughly 9 /2 by 13 inches in dimension, although actuallythe picture shown is usually a little larger since it is customary tocut ed the extreme corners of'a picture. As has been pointed out abovethe lines actuallyshown in a so-called 525-line picture numberapproximately-480,

so that each lineon the maximum size picture which may be displayed on a1.6-inch diameter screen is 8/ of an inch wide. A separation betweenadjacent strips of ,5 of aninch will therefore give a one-half elementseparation, which has been indicated above as the maximumdesirable. Thestrips themselves have, of course, finite thickness, and experiment hasshown that copper strip 3 mils in thickness is appropriate for formingthe grating. Subtracting the thickness of the strips from the 10 miltheoretical separation gives an actual separation of 7 mils, and stripsof an inch wide. With such stripsa satisfactory grating may be formed byany of the methods next to be described.

One out of several methods of constructing the grating is illustrated inFigs. 2, 3 and 4, still in semi-diagrammatic form, the dimensions againbeing exaggerated in order to show more clearly the actual construction.Where this structure is used the strips 27 and 29 which form the gratingare laced or woven together by means of fiber glass cords 37 which arelaced around the strips at intervals along their length. In the showingof Fig. 3 only two of these cords are shown but as the gratings areactually constructed they are placed at intervals along the strips whichare of the same order of magnitude as the spacing between the strips.The ends of the strips are set in slots formed at insulated end supports39. The strips are slightly staggered lengthwise so that strips 27project beyond strips 29 at one end where they are electricallyconnected to the lead 31, while at the other end the strips 29 projectand are connected to the lead 33. The same convention as before is usedto indicate the different colored phosphors.

Figs. 5, (Sand 7 show alternative methods of fastening the stripstogether'intoa continuous grating. In the method shown in Fig. 5 notches41 are formed at in tervals along the strips. Similarly notchedinsulating blocksor separators 43 are positioned between the strips withnotches alined with the notches in the strips themselves, and bindingcords of fiber-glass 45, positioned by the notches, fasten the wholestructure together. The separators are slightly wedge shaped to give theproper tilt to permit parallel entry of the beam, but the angle betweenany two successive strips is too small to show in the figures.

In the modification shown in Fig. 6 transverse corrugations 47 areformed in the strips, preferably before the deposition of the phosphorsupon them. Insulating beads 49 are fused to these corrugations and serveto determine the relative positions of the strips. As before, the endsof the strips aresecured to support rods 39.

In the method of construction shown in Fig. 7 the strips areperforatedat intervals and glass threads 51 are passed through the perforations.Glass beads 53 are strung on these threads between the strips to act asseparators. The tilt to permit parallel entry of the beam is secured byusing beads of slightly different diameter at front and rear of thestrips.

In the construction shown in Fig. 8 the strips themselves are similar inform to those of Fig. 6, but instead of being linear they are wound intoa spiral form. This construction has a considerable advantage from thepoint of view of rigidity and ease of manufacture but because of aconsiderable inductance and distributed capacity in .the structure as awhole the rate at which the color can be switchedis .limited. .Thestructure of Fig. 8 is practicalfor either field sequential or linesequential. methods of transmission but mayor may not be suitable fordot sequential systems, depending upon the rate at which the colors arechanged.

Fig. 9 shows a modificationof the device wherein the strips arewedge-shaped instead of flat as in the other modifications which havebeen described. The wedgeshaped strips 27' and 29 are mounted with theirapices facing the translucent target 23 and their bases toward theelectron gun. The grating can. be formed by sub stantially any of themethods that have been described for fiat strips. This structure has thedisadvantages that it cuts off a greater portion of the electron beam,the bases of the strip acting as a diaphragm which intercepts aconsiderable number of the electrons, and, further, that a highervoltage is required to deflect that portion of the beam which does passthrough the grating and cause it to strike entirely against the properphosphor. The construction has the countervailing advantage that thetransfer of light from the phosphors on the strips is more effectivethan in the other cases mentioned.

Still a further modification is shown in Fig. 10. Here again the strips27 and 29 are slightly wedge-shaped in form, but in this casetransparent glass beads 56 are fused to the apices of the wedges and thephosphors are deposited upon these beads. With this construction thebeam does not have to be deflected so that it actually hits against thesides of the strips, but only far enough so that it hits the phosphors.As in the case of the modification last described the optical efiiciencyis somewhat higher than in the case of the flat strips. Control voltagesare also required to be somewhat higher than before, but not quite ashigh as in the case of the modification of Fig. 9.

Whatever construction is used for the grating the electrical effect ismuch the same. What happens is, perhaps, best shown in Fig. 11. In thisfigure the dotted line 55 indicates the path of the beam when the stripsof grating are at the same potential as the target 23 itself, the beampassing undeflected through the grating and exciting the luminescenttarget only. When a control voltage is applied to the gratings throughthe leads 31 and 33 electric fields are set up between each adjacentpair of strips. Fig. 11 shows the strips 27 as positive and strips 29 asnegative. The fields between the strips are oppositely directed asbetween successive pairs of strips; i. e., a field between the upperstrip 291 and strip 271 is directed (conventionally) downward, while thefield between strips 271 and 292 is directed upward. As a resultwhichever side of strip 271 the beam may fall, it is deflected towardthat strip as shown by the dotted lines 55' and if the control voltagebe properly chosen with respect to the strip separation and beamvelocity it will strike on that strip only and excite only thecorrespond ing phosphor. If the strip sizes and separation are of themagnitude here recommended the beam will strike at least two strips ofthe same color phosphor, and the light falling on and transmitted by thetranslucent target will be all of one color. Reversal of the controlvoltage will change the color from red to blue or vice versa as the casemay be.

Since the spacing between the strips is of sub-pictureelement dimensionsthe strips do not injure the definition procurable from the tube. thatthe colors when reproduced are in superimposed position with respect tothe observer, rather than adjacent one another as with many of thesuggested tube forms.

One of the major advantages of the tube of this invention is that nonecessity arises for having the beam follow the strips. Wherever it mayfall it will produce a spot of light of the proper color and ofapproximately the dimension of the cross-section of the beam. The stripsmay therefore be mounted either parallel to, transverse to or diagonalto the scanning line without any material effect upon the picture. It isthis fact also which makes the spiral construction of the screenposible. In order that the deflecting potentials may always be equallyeffective and to prevent distortion of the spot shape and size it isdesirable that the distances between successive strips be substantiallya constant, i. e., that the strips be parallel, either in the rigoroussense of that term or substantially concentric.

Throughout this specification it has been assumed that the greenphosphor would be the one to be deposited directly upon the transparenttarget and that red and It also must be emphasized,

blue phosphors would be deposited on the grating strips. Thisarrangement is chosen purely from practical consideration; theoreticallyphosphors emissive of any three colors, none of which can be formed byadditive combinations of the other two, may be used and it is immaterialwhich of these phosphors be deposited upon the target itself and whichupon the grating strips. Actually it may be shown that the best colorfidelity can be secured by the use of red, green and blue phosphors, andbecause of the much greater sensitivity of the eye to green light thanto light of the other two primary colors detail is much more visible ingreen than it is when depicted in the other primary colored lights.

Light emitted from the grating strips is somewhat scattered, althoughnot seriously so. It has been shown by other experimenters that if thedetail be sharply depicted in green a considerable lack of resolution inthe red and the blue may be easily tolerated without perceptibledegradation of the image as viewed by the eye. This is the reason forthe choice of the phosphor deposition here recommended but satisfactoryresults can be obtained by other depositions of other primaries as hasbeen indicated above.

It was stated in the first part of this specification that the tube ofthis invention is applicable not only to field, line or dot sequentialsystems but also to simultaneous systems of television transmission. Itis believed that the method of application to any sequential system isself-evident. Its use for simultaneous systems involves the reception ofthe three simultaneously transmitted color signals and their switchingas between the target and the strip areas at a high rate which isindependent of any transmitted signal but which is determined within thereceiver itself. This method of simultaneous reception is covered by acopending patent application filed simultaneously with this; suchsimultaneous method of reception is not considered to be a part of thisinvention but is mentioned here merely to emphasize the versatility ofthe tube of this invention.

A few of the points which have merely been alluded to in the foregoingdescription may perhaps need some further explanation. It has beenstated that the distance between the separators between the gratingstrips should be of the same order of magniude as the separation betweenthe strips themselves. This serves to confine scattered light andincrease definition, and is desirable irrespective of the type ofseparator used.

Moreover,although it has been mentioned that the deflecting voltagesapplied between the grid strips is not critical, it should be noted thatany voltage above the minimum defined by the strip separation willsecure the desired result, excess voltage merely shifting slightly thepoint on the strips from which maximum light emission occurs. Byappropriately grading the density of the phosphors the loss of lightcaused by the greater distance of this point from the screen may becompensated.

Finally, it should be pointed out that although the drawings show anactual electrical connection to the target 23, this connection can beomitted as it frequently is in tubes of ordinary construction assecondary emission from the screen will result in its finding a propermean potential. Its omission may, in fact, ofler a definite advantage inthat even the thinnest conductive coating will result in a loss of lightthrough the screen.

I claim:

1. A cathode-ray tube for the production of television images inpolychrome comprising an envelop, an electron gun within said envelopfor generating a beam of cathode rays, a target area within saidenvelope formed of transparent material and positioned to receive saidbeam of cathode rays, a translucent coating of a phosphor emissive oflight of one primary color on said target area, a grating comprising aplurality of mutually insulated conductive strips mounted edge-onadjacent to said translucent coating, said strips being wedge-shaped incross section Withthebases, of .the wedges directed toward thecathode-ray gun and the apices of the wedges directed toward the targetarea, a coating of aphosphor .ernissive of .light of a difierentprimarycolor from that of said first mentioned coating on each of saidtstrips,and electrical connections to all of said strips for applying deflectingpotentials therebetween.

2. A cathode-ray tubevfor the production of television images inpolychrome comprising an envelop, anelectron gun Within said envelop forgenerating a beam of cathode rays, a target area within said envelopformed of transparent material and positioned to receive said beam ofcathode rays, a translucent coating of a phosphor emissive of light ofone primary color ontsaid target area, a grating comprising a pluralityof mutually insulated conductive strips mounted edge-on closely adjacentto said translucent coating, glass beadssecured to the edges of saidstrips comprising the grating which are apposed to the target area.

3. The cathode-ray tube claimed in claim 2 wherein the glass heads attheregion of beam entrance andbeam 1 2 exittof the gratingstripare ofdifferent diameter thereby permit para1lel:entry of the. beam.

4. The cathode-(ray tube claimed in claim 2' wherein phosphor coatingsaredeposited uponthe beads.

References Citedin the tile of this patent UNITED STATES PATENTS2,307,188 2,446,249 Schroeder Aug. 3, 1948 2,446,440 Swedlund Aug. 3,1948 2,461,515 Bronwell Feb. 15, 1949 2,498,705 Parker Feb. 28, 19502,518,200 Sziklai et'al Aug. 8, 1950 2,529,485 Chew Nov. 14, 19502,571,991 Snyder, Jr Oct. 16, 1951 2,579,705 Schroeder Dec. 25, 19512,635,203 Pakswer Apr. 14, 1953 FOREIGN PATENTS 443,896 Great BritainMar. 10, 1936 Bedford Jan. 5, 1943

